High-Performance and Broad-Viewing-Angle Structural Colored Films with Carbon Black and Carbon Quantum Dot Doping

Abstract

Traditional photonic crystal films are becoming increasingly popular in the fields of smart sensing and optical devices due to their high brightness level, but at the same time there is a lack of color saturation and angle dependence, which seriously affects the application of structural color in visual response and display fields. Here, the optical performance of the photonic crystal films was improved by doping a certain amount of carbon black and carbon quantum dots during the film preparation process. Doping carbon quantum dots can effectively compensate for the optical brightness loss caused by the introduction of black substances by utilizing their self-sustained photoluminescence behavior that matches the photonic bandgap. In addition, the introduction of black nanoparticles can effectively enhance surface-resonant scattering, resulting in low angle-dependent structural colors, further expanding the application of structural color in optical display fields.

1. Introduction

Structural colors originate from the optical phenomenon that arises from the delicate interactions between natural intricate structures and light, including selective reflection, scattering, and diffraction [1,2,3,4]. Unlike dyes, pigments, or metal coloring mechanisms that involve energy dissipation, this physical coloring mechanism has unique advantages such as environmental friendliness and non-fading, and has become closely related to the rapidly growing photonics field [5,6]. It is widely used in sensing and display, anti-counterfeiting encryption, etc. [7,8]. Currently, the artificial construction of high-performance, high-strength structural color materials is becoming increasingly important. The most common type is photonic crystals (PCs), which are formed by periodically alternating materials of different refractive indices (or dielectric constants) in space. When the lattice parameter is the same as the size of the visible light wavelength, it displays a vibrant structural color. Furthermore, due to the Bragg diffraction phenomenon, its color often changes with the observation angle, presenting iridescent colors. Because of its unique optical properties, and excellent color brightness, it has become the main type of optical material with structural color. However, the structural color saturation of the photonic crystals needs to be further improved, and it is usually limited in its application in the sensing field due to its dependence on the angle [9,10,11]. It is difficult to adapt to the field of color perception, and iridescent color changes with the angle will greatly hinder the application of structural color in the sensing and display fields. For example, in the analysis sensing field, observers will be confused by the interference of angle-dependent colors and the color changes caused by the lattice spacing change, so the observation is often fixed at a specific angle.

Therefore, researchers are seeking more ways to solve this limitation, such as using unconventional preparation methods to break the crystallization trend and preparing amorphous photonic structure films to achieve angle-independent colors [12,13,14,15], but in exchange, the amorphous structural colors produced are somewhat dull, and black substances need to be introduced to improve the structure color. The absorption properties of the black substances are used to suppress their non-coherent light scattering, and the brightness is greatly reduced. In addition, choosing to dope small nanoparticles of uniform size that do not change the regularity of the ordered optical structure to suppress its resonant scattering and improve the structure color brightness [16], such as how introducing black nanoparticles below 50 nm into the voids of the face-centered cubic lattice without affecting the lattice quality significantly changes the perceived color, expands the viewing angle, eliminates the strong dependence of the perceived color on the light source position, and greatly enhances color saturation, but the structural color brightness needs to be further improved. Additionally, researchers choose to construct multi-layer heterogeneous structures to achieve an ordered interface with a wide field of view, such as by weakening the optical angle dependence by utilizing the multi-layer optical effects of ordered–disordered hierarchical structures [17], or by increasing the effective refractive index by selecting high refractive index materials, eliminating scattering and background light interference, to achieve non-iridescent structural colors [18]. However, these methods have complex assembly processes and stringent requirements for the selection of assembly particles. Moreover, although the absorption and fluorescent enhancement of many luminescent materials that have been doped into photonic crystals have been proved, it remains a challenge to simultaneously improve the brightness and saturation of structural colors across the entire visible spectrum. Based on this, we expand the bandgap width by simply doping carbon black and utilizing the absorption of non-coherent scattered light by the black material to effectively improve the saturation of the photonic crystal. We also introduce carbon quantum dots to utilize the wavelength-tunable photoluminescence that matches the photonic bandgap to effectively compensate for the color brightness of the structural color. To further improve the structural color stability and mechanical properties of the film, we fill the interface of the film with polyacrylamide hydrogels to construct novel high-brightness photonic crystal humidity sensors that utilize the visual monitoring of the volume change in the hydrogels after absorbing moisture to monitor the humidity level. In summary, this research on reducing the interference of observational angle changes on the response signals caused by external stimuli will promote the practical application of structural color in decoration, sensors, displays, or other color-related fields.

2. Materials and Methods

2.1. Materials

Styrene (St), methyl methacrylate (MMA), acrylic acid (AA), ammonium persulfate, ammonium bicarbonate, sodium alkyl benzene sulfonate, and acrylamide, were purchased from the Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China); N, N′-methylene-bisacrylamide (BIS) and 2,2-dimethoxy-2-phenylacetophenone (DEPO) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China); and carbon quantum dots (CQDs, XF353) and carbon black (CB, XFI15) were purchased from Xianfeng Nanomaterials and Technology Co., Ltd. (Nanjing, China). All chemicals were used without further purification.

2.2. Preparation of High-Performance Photonic Crystals

The selected photonic crystal templates were assembled with colloidal particles into a core–shell structure. By adding emulsifier, sodium alkyl benzene sulfonate (5–20 g), styrene (19 g), methyl methacrylate (1 g), and acrylic acid (1 g) to a fixed system (100 mL), with the aid of ammonium persulfate as an initiator, the monodisperse, uniform-sized hydrophilic polystyrene colloidal particles were obtained through a one-step emulsion polymerization method [P(St-MMA-AA)] [19]. No further purification was needed, and the particle concentration was approximately 10%. The colloidal particles were diluted to 0.2% and sonicated before being placed in 5 mL small beakers. After 2 min of further sonication, CB and CQDs were added, and the mixtures were sonicated again for 2 min. Taking the 40 g polystyrene emulsion system as an example (10 μL CB + 50 μL CQDs), the glass slide was cleaned with piranha solution, and the colloidal particles were deposited vertically onto the glass slide in a self-assembly process from bottom to top for 24 h at a constant temperature and humidity (60 °C, 60%). During the assembly process, brilliant and high-saturation structural color films were obtained.

2.3. Preparation of the High-Brightness Photonic Crystal Hydrogel Sensor

A water-based single-component polymer solution was prepared by mixing acrylamide monomer (15 wt%, 30 mL), photoinitiator DEOP (10 μL), and crosslinker Bis (3%, 45 mg) in deionized water. The doped photonic crystal template was vertically inserted into the single-component polymer solution for 15 s, and the operation was repeated twice to allow the single-component solution to uniformly penetrate the gaps between the template assembly particles. After removing the photonic crystal template, a cover slip was applied to remove air bubbles, and the sample was placed under a UV lamp for 10 min (365 nm) to initiate polymerization. The cover slip was then removed to obtain a doped high-performance photonic crystal sensor with optical properties. The humidity response experiment was conducted under a constant temperature and in a humidity chamber. At a constant temperature, the humidity range was adjusted (50%, 65%, 85%, 90%, 100%), and the sample was placed in the chamber for one hour to respond to the humidity. After one hour, a photograph was taken at each humidity condition, and the corresponding reflection peak position was recorded. By repeating the humidity conditions, the durability and optical sensor repeatability were characterized.

2.4. Characterization

The photos of doped photonic crystals (DPCs) were taken with a digital camera (Nikon, Tokyo, Japan, D5600) under diffused light. The reflection spectrum of DPCs was measured by an HR fiberoptic UV–vis spectrometer (Avantes, Almere, The Netherlands, AvaSpec-ULS3648-USB2, resolution ratio: 2.2~2.4 nm) in the reflection mode via a 600 μm broadband optical fiber (Avantes, Almere, The Netherlands, Fcr-7UVIR-2-ME) at various and normal incident angles relative to the normal. The microstructures of the DPCs were characterized by SEM (Hitachi, Tokyo, Japan, SU8600), and CQDs used for doping were characterized by TEM (JEOL, Tokyo, Japan, JEM-F200). Colloidal particle assembly and humidity control were used for the constant temperature and humidity chamber (Yiheng, Shanghai, China, LHS-80HC-I).

3. Results

3.1. The High Optical Performance of Photonic Crystal Film Doped with CB and CQDs

As shown in Figure 1a, by relying on the electrostatic repulsion of the hydrophilic mono-disperse colloidal particles, a periodic photonic crystal structure could be achieved easily on the glass sheet (thickness: 1 mm) and revealed red, green, and blue structural colors. The nominal thickness of the PCs (photonic structures) after polymerization was 150 μm. Different structural colors were obtained by adjusting the size of the assembled particles (187 nm, 225 nm, 273 nm). The color saturation of un-doped PCs was low and the color was pale. The high-saturation structural color was achieved on the doped PCs (DPCs). Due to carbon black reflect very little light in the visible light region and having stable properties that are less prone to oxidation, it can uniformly absorb the incoherent scattering light to reduce the background peak of the reflectance spectrum and enhance the structural color of the DPCs. However, the introduction of carbon black inevitably leads to a decrease in the brightness of the structural color. As is well known, the photoluminescent behavior of CQDs (luminescence excitation spectrum range: 380–560 nm) varies with the excitation wavelength [20]. The quantum dot particles embedded in the gap of the photonic crystal can take advantage of the wide wavelength-tunable photoluminescence of the quantum dots to effectively compensate for the brightness of the structural color.

From Figure 2a,b, the reflectance spectrum shows that after the introduction of CB, the bandgap half-width of the prepared photonic crystal became wider due to the absorption of non-coherent light by CB itself. By adding CQDs and CB to the existing black light-absorbing substance, it can be found that the reflectance peak of the photonic crystal was enhanced compared to that when only adding carbon black. Quantum dots with sizes less than 10 nm do not affect the crystal lattice quality (Figure S1), and by utilizing the photoluminescence properties of quantum dots, the light emission can be modulated by the interaction between the excitation wavelength and the photonic-structure-induced photonic bandgap, achieving automatic wavelength matching between the quantum dot emission and the photonic bandgap. Based on the slow photon effect, the selective absorption of the blue band edge of the photonic bandgap is enhanced, leading to an increase in luminescence intensity, which effectively compensates for the brightness of the non-rainbow structural color that covers the entire visible region. Usually, the structural color appearance is enhanced by the photonic bandgap, so compared to simply adding carbon black nanoparticles to adjust the saturation of the photonic crystal, the introduction of quantum dots can increase the reflectance peak by 1.7 times.

3.2. Non-Iridescent Structural Colors of Doped Photonic Crystal Films

Fixed photonic crystal films were adjusted to observe angles relative to the normal (0°, 15°, 30°, 45°), and as the angle between the observation path and the normal increased, there was no significant color change in the DPCs, showing a low angle-independent structural color (Figure 3a). These structural films were obtained by adjusting the diameters of the assembled particles (187 nm, 225 nm, 273 nm), and doping the same mass fraction of CQDs and CB. By changing the detection angle of the optical fiber probe, the reflectance spectrum was tested under different angular conditions and it was found that the reflective peaks (525 nm) remained almost unchanged on a typical DPC film with a green structural color, as shown in Figure 3b,c. Only when the detection angle increased to 60° did the reflection peak undergo a slight blue shift. This further proves that this is different from the traditional Bragg diffraction-induced iridescent color, which exhibits low-angle dependence and is more suitable for applications in sensing and response fields. This is mainly due to the low concentration of CB nanoparticles that are incorporated, which can be embedded effectively in the gaps between colloidal particles to enhance the resonant scattering of the surface layer while preserving the initial periodic ordered assembly [16]. Due to coupling with standing waves produced by multiple reflection, the cross-section scattering depending on the Bragg resonance is enhanced and anisotropic. In short, scattering is strongly enhanced under Bragg conditions, providing very wide viewing angles for structural colors.

3.3. High Optical Performance of Doped Photonic Crystal Hydrogel Humidity Sensors

High-performance structural colors achieved by doping are conducive to applications in sensing and display fields. With their non-iridescent characteristics, they can effectively eliminate the interference caused by the changes in structural color resulting from different observation angles. As shown in Figure 4a, DPC–hydrogel composite films were prepared via the templating approach. The hydrogel monomer solution was introduced into the film gaps using a repeated vertical lifting method to avoid excessive PAA coverage on the DPCs, which affects optical performance. Under ultraviolet light irradiation, polymerization was carried out to form a DPC–hydrogel composite sensor. As the relative humidity (RH) changes, the DPC–hydrogel nanocomposite film undergoes expansion and contraction as a result of its swelling and deswelling properties, and the lattice spacing changes accordingly, thereby realizing the colorimetric sensing of the hydrogel humidity response. As shown in Figure 4b,c, as the humidity increases from 50% to 100%, the structural color also changes from the corresponding blue to red. The reflectance peaks of the DPC–hydrogel composite sensor showed red shifts from 419 to 623 nm. It is worth noting that to achieve structural color changes that cover the entire visible light region with changes in humidity, the initial DPCs were preferentially selected with a blue structural color, and the template photonic bandgap is in the near-ultraviolet light region (diameter: 175 nm). By controlling the appropriate amount of hydrogel monomers, the initial blue DPC–hydrogel can be achieved. Due to the filling of the hydrogel, the refractive index of the PCs changes from 1.6 to 1.46, and at the same time, the humidity response proceeds. After swelling fully, the refractive index will further decrease (to 1.32), and thus the doped CB and CQDs can effectively ensure the optical performance to better respond to visual stimuli from the external environment. Furthermore, after undergoing six humidity-reversible cycles, the band gap values and optical performance can be maintained after 3.5 min per cycle (Figure 4d), indicating that there is good stability and durability in the sensing and display platform.

4. Conclusions

In summary, carbon black nanoparticles and carbon quantum dots are doped into photonic crystal films to prepare novel brilliant and highly saturated structural color films. Carbon black can effectively absorb incoherently scattered light, and the introduction of quantum dots can effectively compensate for optical brightness by matching its photoluminescence and photonic band gap. More importantly, based on resonance scattering, this photonic crystal film exhibits a broad viewing angle property, thus indicating this high-performance structural color film for humidity response to further promote optical sensing, anti-counterfeiting encryption, and other applications.